In a wireless sensor network, since an unlicensed third-party sensor node may access the wireless sensor network and illegally hack data transceived between legal sensor nodes, safe transception of data between the sensor nodes is a very important issue. Especially, in the wireless sensor network, a sensor node transmits a signal in all directions using a directive antenna (that is, transmits the signal in a broadcasting way), an unlicensed third-party may be able to easily hack data transceived between the sensor nodes. For safe transception of data, typically, the data transceived between the sensor nodes are encrypted. Conventionally, there have been used various methods for generating secret values to be used between the sensor nodes for the encryption of data and distributing the generated secret values safely.
One of such conventional methods is using a public key authentication center connected to the wireless sensor network. The public key authentication center generates a private key and a public key of each sensor node and distributes the generated private key and public key to each sensor node. One sensor node may encrypt data by using a public key of the other sensor node received from the public key authentication center and transmits the encrypted data to the other sensor node. The other sensor node that has received the encrypted data may decrypt the data by using its private key. In the secret key generation and distribution technique based on the public key authentication center, however, it is essential to additionally provide the public key authentication center. Further, in this method, in order to send data from one sensor node to the other, the one sensor node should have the public key of the other sensor node. However, in consideration of the characteristic of the wireless sensor network that is randomly distributed in a region where people is difficult to access, it would be difficult to embody this technique due to the difficulty in communications between the wireless network and the public key authentication center. Further, it would also be difficult to embody this technique in sensor nodes equipped with a low energy source.
To solve the aforementioned problems, in another conventional method, a secret key is generated by using a physical layer characteristic of a wireless communication channel between sensor nodes. Referring to FIG. 1, a signal transmitted from a sensor node A to a sensor node B is sent in a broadcasting way, and the transmitted signal is delay-spread according to a communication channel environment, i.e., a physical layer characteristic between the sensor node A and the sensor node B and then received by the sensor node B. Accordingly, even in case a same signal is transceived between sensor nodes, the signals actually received by the respective sensors are all different.
By way of example, even in case the sensor node A and a sensor node C transmit the same signal to the sensor node B in a broadcasting way, the transmission signal broadcasted from the sensor node A is received by the sensor node B as a signal 1, a signal 2 and a signal 3 through different delay spreads according to a physical layer characteristic between the sensor node A and the sensor node B. The transmission signal broadcasted from the sensor node C is received by the sensor node B as a signal 1′, a signal 2′ and a signal 3′ through different delay spreads according to a physical layer characteristic between the sensor node B and the sensor node C.
The sensor node B may generate data of the physical layer characteristic between the sensor node B and the sensor node A according to the delay spread patterns of the signals 1, 2 and 3 received from the sensor node A. The generated data of the physical layer characteristic may be used as a secret key between the sensor node A and the sensor node B. Likewise, the sensor node B may generate data of the physical layer characteristic between the sensor node B and the sensor node C according to the delay spread patterns of the signals 1′, 2 and 3′ received from the sensor node C. The generated data of the physical layer characteristic may be used as a secret key between the sensor node C and the sensor node B.
FIG. 2 illustrates secret keys generated based on physical layer characteristics between sensor nodes in a multi-hop wireless communication network.
Referring to FIG. 2, a sensor node A and a sensor node B generate a secret key SAB based on a physical layer characteristic therebetween and share the generated secret key SAB. The sensor node B and a sensor node C generate a secret key SBC based on a physical layer characteristic therebetween and share the generated secret key SBC. The sensor node C and a sensor node D generate a secret key SCD based on a physical layer characteristic therebetween and share the generated secret key SCD.
When transmitting data from the sensor node A to the sensor node D while encrypting the data using the secret keys generated based on the physical layer characteristics in the multi-hop communication network, the sensor node A transmits the data to the sensor node B after encrypting the data with the secret key SAB between the sensor node A and the sensor node B. The sensor node B decrypts the received data with the secret key SAB. Then, the sensor node B encrypts the decrypted data again with the secret key SBC and transmits the encrypted data to the sensor node C. The sensor node C decrypts the received data with the secret key SBC. Then, the sensor node C encrypts the decrypted data again with the secret key SCD and transmits the encrypted data to the sensor node D. The sensor node D decrypts the received data with the secret key SCD.
That is, the secret keys generated based on the physical layer characteristics may be different between the sensor nodes. In the multi-hop wireless communication network, whenever data is hopped, the data should be encrypted and decrypted with secrets keys of intermediate sensors in a routing path repeatedly. Accordingly, when transceiving data in the multi-hop wireless communication network using the secret keys generated based on the physical layer characteristics, a great amount of energy would be consumed for the encryption and decryption of the data. Thus, it may be difficult to apply this technique to a sensor node having a limited energy source.
Furthermore, in a mobile sensor network where sensor nodes are freely movable, in case a sensor node moves to another position from its original position, a secret key generated based on a physical layer characteristic between that sensor node and the other sensor is also changed. Thus, whenever the sensor nodes move, new secret keys need to be generated.